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Aggregates of α-synuclein are the principal component of Lewy bodies, neuronal inclusions found in Parkinson and other neurodegenerative diseases—and they get around. Cells normally keep their α-synuclein problems to themselves, but when more accumulates than they can handle, they may release the protein into the extracellular space. Unfortunately, dumping trash in your neighbor’s yard is rarely a good idea. A report posted online in PNAS July 27 shows that the nearby cells can pick up α-synuclein jetsam, helping the toxic protein spread from cell to cell across the brain. The results may help explain why tissues transplanted into people with Parkinson’s eventually succumb to the disease (see ARF related news story; Kordower et al., 2008 and Li et al., 2008), and they beef up the idea that aggregation-prone proteins can be transmitted in the brain much like prions.

The work was a joint effort between the laboratories of Seung-Jae Lee of Konkuk University in Seoul, South Korea, and Eliezer Masliah of the University of California at San Diego. The joint first authors were Paula Desplats from the University of California and He-Jin Lee from Konkuk University. PD Online Research coverage carries commentary on the discovery and an interview with Lee.

Lee and colleagues had already discovered that neurons can exocytose (Lee et al., 2005) and endocytose (Lee et al., 2008) α-synuclein. In the current work, they put the two together to show that toxic forms of the protein move from cell to cell, seeding new aggregates as they go. In co-cultures of donor neurons expressing human α-synuclein and acceptor stem cells lacking it, it took only a day for nearly half of the acceptors to exhibit α-synuclein accumulation. In human α-synuclein-expressing mice that received stem cell grafts, the transplanted cells also picked up the protein, similar to what may have happened to cell grafts in human clinical trials for PD (see ARF related news story). One month after the mice received transplants, 15 percent of grafted cells contained the human protein, and a few hosted Lewy-like inclusion bodies as well. Acceptor neurons exhibited symptoms of apoptosis, including fragmented nuclei and caspase 3 activation, that control cells not exposed to α-synuclein did not.

“This is an interesting and well-done study showing that pathogenic α-synuclein can be transferred from cell to cell both in vitro and in vivo,” wrote Lary Walker of Emory University in Atlanta, Georgia, in an e-mail to ARF. “It should come as no surprise that grafted cells in Parkinson’s patients succumb to synucleinopathy,” he added. “Transplantation of healthy cells into such a diseased brain is rather like trying to rebuild a burned house while the fire is still raging.” He noted that the experimental systems reported in the paper should be useful for analyzing the problem and looking for therapeutics to circumvent it.

Masliah is thinking along similar lines. “We want to develop cells or grafts that would be resistant to these toxic oligomers,” he said. Such graft cells might be oblivious to their neighbor’s garbage.—Amber Dance

Comments

This paper seems to be interesting, revealing an absolute requirement for intracellular delivery of the fibrillated alpha-synuclein to induce Lewy-body like inclusions. The cell-to-cell communication requires intracellular seeding, which is, however, revealing a pattern similar to prion proteins. Hence the question arises whether alpha-synuclein acts like a prion.

It was a pleasure to read this work from the Masliah/Lee duo on the cell-to-cell transmission of α-synuclein (Desplats et al., 2009). This work shows that α-synuclein can be released from cells and is taken up by the neighboring cell, thereby aiding in a progressive spread of the protein. This work continues Seung-Jae Lee’s previous work showing that α-synuclein could be released (Lee et al., 2005) and taken up in neurons (Lee et al., 2008; Lee et al., 2008). While the exact mechanism of the release is currently not well defined, this group has done elegant cell biology work to study the internalization mechanism. They show that fluorescently labeled, recombinant α-synuclein is internalized from the extracellular lumen via a dynamin-1-dependent pathway in vitro. This also occurred in vivo, where injection of GFP-labeled mouse cortical neuronal stem cells into the hippocampus of α-synuclein-transgenic mice led to the efficient uptake of the host α-synuclein into the grafted stem cells after just four weeks. These data add yet another case to the growing list of host-graft relationships in amyloid disorders.

Desplats et al. go on to demonstrate that the internalized α-synuclein promotes inclusion body-like structures in the cytosol of the acceptor cells that are positive for ubiquitin and thioflavin S. This somehow depends on the integrity of lysosomes, as lysosomal pH disruption using v-ATPase inhibitors aggravates the inclusion body formation. How lysosomal function is coupled to a process that occurs in the cytosol is not clear, but the authors rule out membrane leakage. The last figure in the paper gives the take-home message, where the authors demonstrate neurodegeneration in both primary neurons and the mouse cortical stem cells that were exposed to neuronal cell-derived extracellular α-synuclein. This suggests that the released and internalized α-synuclein could be responsible for progression and pathology.

Seeding and spreading are not new features of neurodegenerative diseases (Aβ, e.g., Meyer-Luehmann. et al., 2006; prions, e.g., Bolton et al., 1982). But this time it is different. Three cytoplasmic proteins, as opposed to lumenally exposed membrane proteins, have been recently documented to show this effect. Work from Lee’s group has shown that α-synuclein can be exocytosed from cells. And recently, tau, another cytoplasmic protein of immediate interest to this community because of its involvement in AD and FTD, was shown also to act from outside (Clavaguera et al., 2009). Moreover, polyglutamine aggregates, which are relevant to Huntington disease, are internalized into the cytosol as well (Ren et al., 2009). Several features in this work open up new questions:

1. How are these cytosolic proteins released to the extracellular space (“cytosol to lumen” paradox)? Is the release regulated? Is it only the aggregated versions of the protein, or could the monomeric proteins also be secreted?

2. How does internalized extracellular α-synuclein end up in the cytoplasm (“lumen to cytosol” paradox)? Desplats et al. show that the internalization occurs via endocytosis, but how does the protein enter the cytosol from the endosomes? The authors show that the internalized α-synuclein initiates a Lewy body-like inclusion in the cytosol.

3. In the case of the α-synucleinopathies, the pathology spreads progressively to remote areas of the brain. This work could contribute to the direct understanding of this spreading. On the other hand, in tau pathology, similar spreading occurs (from the transentorhinal cortex to the hippocampus), and again a cytoplasmic protein is released or injected that contributes to the spread of the pathology.

4. How does the spreading occur? Nanotubes could be one way. Prions hijack tunneling nanotubes during their intercellular spreading (Gousset et al., 2009). Though Desplats et al. show that recombinant α-synuclein (and also Ren et al. in the case of polyQ) could enter cells from the extracellular space, nanotubes may aid in the spreading and progression.

5. That a toxic protein would instruct amyloid formation in the wild-type counterpart (be it in the host or graft) on one hand is alarming as it raises caution about stem cell therapies. But on the other, it unfolds a new paradigm in the amyloid pathogenesis. An insightful commentary by Adriano Aguzzi (2009) suggests that many proteins could act like prions; Aguzzi calls them prionoids. Certainly a body of recent work points in this direction. Such studies could be extended to other neurodegenerative amyloid disorders to see if this could be generalized.

One thing is clear. The gap between cell biology and neuroscience is narrowing. Exciting times are to come where we will use cell biology not just for basic understanding of these diseases, but, as shown in the current work, also for therapeutic deliberations.